The subject matter disclosed herein relates to a simplified fuel system and, more particularly, to an aircraft engine fuel system with a fuel metering pump and a fuel system with a pump capacity indicator and a controller which schedules fuel flow to an engine using data collected from the pump capacity indicator.
In modern high performance aircraft engines, internal structures may be cooled and lubricated by a circulating flow of oil, which is distributed and collected throughout the main engine structure. Meanwhile, thermal management methods include the rejection of heat from the circulating oil loops into the flow of fuel entering the engine combustion chamber. Here, the fuel flow is used as a recuperative heat sink which incurs few of the penalties of air cooling, but is limited in effectiveness by the maximum temperature tolerable by the fuel. Further effectiveness of using the flow of fuel, is the limitation necessitated by maintaining the fuel flow above freezing to minimize the possibility of ice formation and subsequent entry into sensitive areas such as engine actuators.
Main fuel pumps for aircraft engines have traditionally been fixed delivery, positive displacement type pumps connected mechanically to the rotating engine shaft. As the flow rate from a pump turning proportional to engine shaft speed cannot match the fuel flow requirements of a gas turbine engine operating under a variety of power levels, however, it became common to size the main fuel pump with an excess flow capacity under all engine operating conditions. The fuel systems, therefore, often include a fuel bypass for routing excess main fuel flow back to the low pressure side of the main pump.
The various components of the fuel bypass systems include a series of pumps and pressure regulating valves, such as a metering pump disposed upstream from a minimum pressure and shut off valve (MPSOV). Here, the metering pump produces output fuel at a predefined pressure to match the back pressure created by the fuel nozzles in the combustor. The MPSOV insures that the desired amount of this output fuel enters the engine and creates a leak tight seal when the engine is not in operation. The MPSOV is configured to maintain a constant pressure drop, which assures the pump discharge pressure is based on the engine combustor pressure.